feat: add the :global {} rule support

[illright]

This implements the proposal discussed in Svelte RFCs: sveltejs/rfcs#22 (comment)

<style>
  .scoped-style {}
  
  :global {
    import 'global-stylesheet';
  
    .global-style {}
  }
</style>

This would allow for fine control of what parts of the stylesheet should be made global.

Tests

• Run the tests tests with npm test or yarn test

[illright]

As recently mentioned in the discussion of the RFC, maybe going with :global {} would be better than @global {} for compliance with CSS modules. Join the discussion in the RFC if you think this PR should be changed to use the colon notation instead.

[illright]

I'm not sure why the build is failing. Can't reproduce the issue on Manjaro with the same Node version that is used in the CI. Besides, I didn't even edit the Less files. Any ideas?

[kaisermann]

Hey @illright 👋 First of all thanks for your pull request! I'll give it a look later today or tomorrow to see what's happening with the tests 😁

[kaisermann]

Done! Release in v3.9.0 🎉 . Thank you very much for your contribution 💯